The electrophoretic effect is well known in the art as evidenced by the many patents and articles which describe this effect. In essence, the electrophoretic effect operates on the principle that when certain particles are electrically charged to a particular polarity, the charged particles will migrate away from a surface charged to the same polarity as the particles and toward a surface charged to a polarity which opposite to that of the charged particles. For example, particles which are positively charged will migrate away from a positively charged surface and towards a negatively charged surface.
Display devices which utilize the electrophoretic effect are commonly known as electrophoretic image displays (EPIDs). EPIDs are very well known in the art. The following patents issued to Frank J. Disanto and Denis A. Krusos, and assigned to Copytele, Inc., the assignee herein, are illustrative of such EPIDs.
U.S. Pat. No. 4,655,897 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, describes an electrophoretic display apparatus comprising an XY matrix of grid and cathode lines, an anode electrode spaced from the grid and cathode matrix, and an electrophoretic dispersion. The patent describes techniques for making such displays as well as suitable dispersions for use with such displays.
U.S. Pat. No. 4,732,830 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, describes methods for making electrophoretic displays as well as describing display construction and operation.
U.S. Pat. No. 4,742,345 entitled ELECTROPHORETIC DISPLAY PANEL APPARATUS AND METHODS THEREFOR, describes improved electrophoretic display panels exhibiting improved alignment and contrast with circuitry for implementing the same as well as methods for providing such a panel.
U.S. Pat. No. 4,746,917 entitled METHOD AND APPARATUS FOR OPERATING ELECTROPHORETIC DISPLAYS BETWEEN A DISPLAY AND A NON-DISPLAY MODE, describes various biasing techniques for operating electrophoretic displays to provide writing, erasing as well as operating the display during a display and non-display mode.
U.S. Pat. No. 4,772,820 entitled MONOLITHIC FLAT PANEL DISPLAY APPARATUS, describes methods and apparatus for fabricating flat panel displays employing electrophoretic principles to enable such displays to be biased and driven by additional circuitry.
The electrophoretic fluids used in EPIDs typically comprise white, light, or dark colored dielectric particles which are suspended in an optically contrasting fluid medium which is either clear or dark-colored, depending upon the color of the particles. See U.S. Pat. No. 5,360,689 entitled COLORED POLYMERIC DIELECTRIC PARTICLES AND METHOD OF MANUFACTURE, issued to Hou et al., which describes black electrophoretic and light-colored electrophoretic particles formed from crystalline polymer particles using a dispersion polymerization technique. In accordance with the electrophoretic effect described above, the electrophoretic particles in the suspension liquids are caused to selectively migrate to, and impinge upon, a transparent screen electrode, thereby displacing the fluid medium from the screen and creating the desired image.
EPIDs have many advantages over other types of flat panel displays. One advantage is that EPIDs use materials which are relatively inexpensive and thus, makes them less costly to manufacture. Another advantage of EPIDs is that the image formed on the screen remains even when power is removed. When the electrophoretic particles or dye particles are caused to move to form an image, the image will not erase and remains on the display even upon removing of power. Hence the image must be erased in the same manner as it was created, by application to the device of an electric field of opposite polarity. Thus, EPIDs have a built-in memory in the sense that the images created by the displays do not have to be refreshed such as those images produced by CRT's and other types of displays.
One drawback associated with most prior art electrophoretic displays is that they are monochromatic. This drawback severely limits the number of applications where EPIDs can be employed. Accordingly, there is a need for a multi-color EPID which is capable of reliable operation and which is economical to fabricate.